Linux is almost the only one where getpid() == thread-id. IRIX is

another one, but only if you stick to sproc(2).

doc/crypto/threads.pod

@@ -33,7 +33,7 @@ lock. They can be useful for debugging.
 
 id_function(void) is a function that returns a thread ID. It is not
 needed on Windows nor on platforms where getpid() returns a different
-ID for each thread.
+ID for each thread (most notably Linux).
 
 =head1 RETURN VALUES